Mackay said the acquisition would give the New Zealand firm access to "a wider market than what we have today", especially in the US.
"Intermedix, the acquirer, has the relationships with all the customers we are targeting right now," he said. "It gives us easy entry to the EMS market - EMS meaning ambulance, fire and potentially police."
Optima employs 25 staff at its Auckland headquarters. Another five are employed in the firm's offices in the US and Europe.
Mackay said there wasn't any risk of Intermedix closing down Optima's operations in this country.
In fact, the US company wanted to invest in up to five additional New Zealand-based staff members, he said.
Before the acquisition, Optima's shareholders included Mackay and the company's four founders - David Ryan, Paul Day, Jeff Meyer and Andrew Mason - as well as Sir Stephen Tindall's K1W1 fund and the Ice Angels investment group.
Optima was founded in 1998 by students and staff at the University of Auckland's engineering faculty.